As the molding method of a fiber reinforced thermoplastic plastic, stamping molding is most generally performed in which a substrate (prepreg) of a continuous reinforcing fiber impregnated with a thermoplastic resin is layered and heated and pressed using a press or the like to form into an intended shape. The fiber reinforced plastic thus obtained has excellent mechanical properties since a continuous reinforcing fiber is used. In addition, it is possible to design the required mechanical properties and variations in mechanical properties are also small as continuous reinforcing fibers are regularly arranged. However, it is difficult to form into a complicated shape such as a three-dimensional shape since it is a continuous reinforcing fiber, and thus the use of the fiber reinforced plastic manufactured in this manner is mainly limited to a member having a shape close to a flat shape.
In order to solve this problem, a method to obtain a sheet exhibiting excellent stamping moldability and favorable fluidity by dispersing chip-shaped prepregs obtained by cutting a tape-shaped prepreg having a narrow width into a certain length on a plane (Patent Document 1). However, it is significantly difficult to dispose chip-shaped prepregs having a certain width and a certain length on a flat plate in completely random directions, and thus the sheet manufactured in this manner has a problem that the mechanical properties thereof are different depending on the position or the orientation even in the same sheet.
In addition, D-LFT molding in which the reinforcing fiber is directly fed to the screw portion of a molding machine, cutting and dispersion of the fiber are conducted at the same time, and injection molding or extrusion molding is then continuously conducted is also performed in recent years for the purpose of improving production efficiency (Non-Patent Document 1). According to this method, the reinforcing fiber easily flows and is also able to follow a complicated shape such as a three-dimensional shape since it is cut into a proper length. However, the D-LFT has a problem that the mechanical properties thereof decrease or variations in values of the mechanical properties increase since unevenness in fiber length or fiber distribution occurs in the cutting and dispersing process.
A layered substrate is disclosed that is obtained by carving a slit into a prepreg composed of a continuous fiber and a thermoplastic resin in order to remedy the drawbacks of the materials as described above and thus able to be molded in a short time, exhibits excellent formability at the time of molding, and exerts excellent mechanical properties when being formed into a fiber reinforced plastic (Patent Documents 2 and 3). However, it cannot be said that a strength enough to be applied as a structural material is obtained from this layered substrate although it exhibits higher mechanical properties as compared with the D-LFT and the variations therein are small.
In addition, a method to improve the problem on the strength and the variation therein described above by optimizing the slit shape is disclosed (Patent Documents 4 to 6). However, according to this method, the mechanical properties and the variations therein are improved but uniform fluidity of the layered substrate to be formed into a complicated three-dimensional shape such as a thin rib or boss is insufficient. In addition, according to this method, it is required to dispose a large number of steep slits in the fiber direction and it takes a significantly long time to carve the slit in the case of cutting using a cutting plotter. In addition, according to this method, in the case of disposing the slit by punching, there is a problem that not only the manufacturing cost of the punching blade is enormous but also a crack is likely to generate in the fiber direction at the time of punching and missing of sheet between the adjacent slits occurs.